1. Field of the Invention
The present invention relates generally to a method and apparatus for use in the construction of precast, moment resisting frames of buildings. More particularly the invention concerns a method and apparatus for use in positioning high-strength cables within a precast, moment resisting frame made up of columns and beams that are interconnected by the high-strength cables.
2. Discussion of the Prior Art
In recent years great strides have been made in the design of high rise buildings that resist lateral forces as well as vertical or gravity forces. Lateral or horizontal forces are normally imposed on a building or structure by either wind forces or seismic forces applied to the building. Of particular concern in earthquake-prone areas are seismic forces, and great strides have been made in these areas in the design of seismic-resistant structures. However, experience has shown that even relatively new seismic-resistant, steel-frame buildings have serious shortcomings. For example, building codes are typically written with personal safety in mind and generally require that certain structural members bend to absorb the force of a serious quake and, in this way, spare the occupants of the building. However, following the earthquake, buildings constructed to these codes, while preserving human life may, nevertheless require major repairs, and, in some cases the entire building must be demolished because of the structural damage suffered.
One of the most successful prior art moment resisting frame designs is the design developed by the assignee of the present invention. This novel design concerns precast moment resisting frames made up of columns and beams that are tied together in the horizontal direction by high-strength cables. These cables are entrained through a passageway located in the center of the beam so as to pass through the columns at the same elevation as the beam. In these structures, after the beam and column elements are erected, the cables are entrained through the passageways and stretched or pretensioned. The stretched cables are clamped at the face of the columns resulting in the horizontal force that securely ties the columns and beams together. In some moment frames the horizontal ducts carried within the beams may contain as many as twenty, 0.6-inch-diameter, high-strength cables with a post tensioned force of on the order of 35,000 pounds each. Accordingly, the resulting force acting on the column from the two perpendicular forces transferred to the column may well exceed four hundred tons.
In addition to the high strength cables, the columns and beams of this novel frame design are connected together with reinforcing steel that absorbs energy during lateral movement of the frame. More particularly, at every location where a beam meets a column, short bars or rods, are strategically located above and below the central cable, help secure the joint. Made of stretchy or "mild" steel, the rods uniquely serve to effectively dampen the earthquake's effects.
In an earthquake that causes the building to shake and the vertical columns to sway, the central steel cable of the aforementioned prior art designs will stretch safely and rebound slightly without permitting the beam-to-column joints to shift out of alignment. The mild-steel bars, because of their placement above and below the central cable at each joint, take the brunt of the sideways forces, stretching and retracting much like very large shock absorbers. When the earthquake ends, the frame snaps back to its original shape without major structural damage having occurred.
In the aforementioned types of prior art structures, tension in the stressed cables is typically transferred to the columns through wedge type anchors. The anchor imposes a clamping force on the columns transferring it through the interface between the columns and beam. This creates a compressive force through the moment resisting frame. However, where two perpendicular moment frames intersect at a corner, the bundles of cables from both of the perpendicular beams interfere with each other at the corner column.
A significant problem recognized in the prior art construction concerns the interference of the cables at the corner condition where the five-inch diameter passageways intersect. As will be better appreciated from the discussion which follows, the novel corner transfer, or corner sweep component of the present invention uniquely solves this difficult prior art problem and is specially designed to transfer the forces from the cables into a quarter circle pipe that comprises a part of the corner sweep assembly. The corner sweep assembly uniquely allows the cables to be continued from one moment frame to an adjacent, perpendicularly extending moment frame with the resultant forces being effectively transferred into the interior of the column.
With the novel construction described in the preceding paragraphs, the lower floors of the building can be finished and occupied at a stage when steel structures are still being fitted out with floors and interiors. This makes large areas of the concrete building useful three or four months sooner than portions of a comparable steel frame structure would be.